io/pointer: rename button names to reflect their meaning, not placement

For example, ButtonLeft may be the right-most button for a left-handed user.
Rename the button names to match their intended use.

This is an API change. Use the following commands to update your
projects:

    $ gofmt -r 'pointer.ButtonLeft -> pointer.ButtonPrimary' -w .
    $ gofmt -r 'pointer.ButtonRight -> pointer.ButtonSecondary' -w .
    $ gofmt -r 'pointer.ButtonMiddle -> pointer.ButtonTertiary' -w .

Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/internal/window/os_x11.go

@@ -362,11 +362,11 @@ func (h *x11EventHandler) handleEvents() bool {
 			const scrollScale = 10
 			switch bevt.button {
 			case C.Button1:
-				btn = pointer.ButtonLeft
+				btn = pointer.ButtonPrimary
 			case C.Button2:
-				btn = pointer.ButtonMiddle
+				btn = pointer.ButtonTertiary
 			case C.Button3:
-				btn = pointer.ButtonRight
+				btn = pointer.ButtonSecondary
 			case C.Button4:
 				// scroll up
 				ev.Type = pointer.Scroll